Previous work in this laboratory has begun to define an immunological basis for the increased susceptibility of aged mice to infection with Mycobacterium tuberculosis. Thus the long-term objective of this proposed continuation project is to conduct an in-depth analysis of the nature of this immunological basis in terms of a hypothesized age-related decline in mechanisms of acquired T cell mediated protective cellular resistance. Much of these proposed analyses will consist of low-dose aerogenic infection models, thus simulating poorly understood events in the aged lung such as recrudescence of latent disease, still relatively common in the aged community, and which thus continues to constitute an important health problem within this growing population. Accordingly, the specific Aims are designed to systematically investigate the nature of T cell response in old, tuberculosis-infected mice, from the perspectives of (a) T cells that mediate protective immunity, (b) T cells that mediate delayed sensitivity, and (c) T cells that give rise to a long- lived state of immunologic memory. In order to model events known to occur within the human population, mice will be given low-dose aerosol infections at various ages to simulate primary exogenous exposure, or initially as young animals, in order to simulate age-associated endogenous recrudescence of infection. In these latter animals, the immune status of the aged animal will be studied directly, by observing events within lung lesions, and indirectly, by measuring the response of the animal to a secondary infection with mycobacteria in the popliteal lymph node. The technology used to achieve these goals will consist of in vivo passive cell transfer, in which the capacity of T cell subsets to adoptively confer protection, DTH, or memory immunity, will be elucidated; flow cytometry, in which the accumulation of alpha/beta and gamma/delta T cells bearing markers of interest into sites of infection will be measured, immunohistochemistry, in which the patterns of cellular accumulation will be documented; and various in situ hybridization, calorimetric, ELISA, and ELISA-spot assays, in which the ability of accumulating cells to elaborate important cytokines will be assessed. Using data gathered by these diverse procedures, it is anticipated that new important knowledge will be gained regarding the precise events which occur in the aged animal exposed to virulent pulmonary tuberculosis.